The present invention relates to the field of flame retardation and smoldering suppression and, more particularly, to novel smoldering suppression compositions which can be beneficially utilized in textile applications.
Textiles are an essential part of everyday life and are found, for example, in draperies, cloths, furniture and vehicle upholsteries, toys, packaging material and many more applications. Consequently, textile flammability is a serious industrial concern.
The flammability of textile fabrics is typically determined by the type of fiber of which the fabric is made. Thus, for example, some synthetic fibers, such as melamine, polyaramides, carbonized acrylic, and glass, are inherently flame resistant, whereby others, such as cotton, polyester and linen, can readily ignite. Fabric flammability also depends on fabric characteristics such as thickness and/or looseness.
The term “fiber” as used herein refers to a natural or synthetic filament capable of being spun into a yarn or made into a fabric.
The terms “fabric”, “textile” and “textile fabric” are used interchangeably hereinafter to describe a sheet structure made from fibers.
Several approaches have been proposed heretofore for minimizing the fire hazard of flammable textiles.
One approach involves fiber copolymerization. In this technique several fiber monomers are mixed and copolymerized, thus improving the properties of a certain fiber (e.g., made of a flammable polymer) through the enhanced properties of another fiber (e.g., made of a fire resistant polymer). This technique, however, is limited by the number of existing fire resistant polymers and their properties, and cannot be tailor-made for any substrate or requirements. Furthermore, the monomers forming the different fibers (e.g. forming a flammable polymer or a fire resistant polymer) are not necessarily compatible, for example, with regard to the type of polymerization (e.g., step polymerization versus condensation polymerization and other polymerization types), thus further limiting the applicability of this technique. An additional disadvantage of this approach is the high cost of the fire resistant polymers.
Another approach includes introduction of flame retardants (FR) in or on the fabric. Thus, flame retardants can be incorporated in the fabric either topically or as a part of the fabric.
Methods in which a flame retardant is applied topically suffer the disadvantage of the common need to apply the protective coating (which includes the FR) in large amounts (termed “high add-on”) in order to obtain the required fabric characteristics. Often, such high add-on adversely affects otherwise desirable aesthetical and textural properties of the fabric. Thus, for example, upon application of a FR, fabrics may become stiff and harsh and may have duller shades, and poor tear strength and abrasion properties.
In methods where the FR forms a part of the fabric matrix, the FR, mixed with the fiber and other possible additives, is subjected to at least some of the processes involved in the fabric manufacture. These processes include, for example, extrusion or injection molding of the fibers. These techniques have many drawbacks, including, for example, the degradation of the FR agent due to the high extrusion temperatures; a reaction of the FR agent with the extruded fiber, which results in a modification of fiber properties such as fiber dyeability, fiber processability or other physical properties of the fiber; and a reaction of the FR agent with the various polymer additives, such as dyes or catalysts, which may also adversely affect the fiber properties and further require the use of large amount of FR.
In addition to the limitations associated therewith, the presently known methods for minimizing textile flammability do not necessarily provide a technical solution to the smoldering (after-flame burning) of fabrics.
Fabric smoldering is an acute problem, and is particularly critical in fabrics that contain a high ratio of cellulose (such as, for example, cotton, viscose, linen or other vegetable fibers).
While textiles may be resistant to open flame burning, the smoldering (also termed “after flame”), which may persist after the open flame has been extinguished, can eventually lead to complete digestion of the fabric (see, for example, “Toxicological Risks of Selected Flame-Retardant Chemicals-2000”, Donald E. Gardner (Chair), Subcommittee on Flame-Retardant Chemicals, Committee on Toxicology, Board on Environmental Studies and Toxicology, National Research Council). Obviously, this leads to failure in many standard flammability tests (see, for example, U.S. Pat. Nos. 3,955,032 and 4,600,606; and V. Mischutin, “Nontoxic Flame Retardant for Textiles” in J. Coated Fabrics, Vol. 7, 1978, pp. 308-318).
Although one solution to this problem is coating the textile fabric with an impermeable material, the feel of such a product is greatly damaged.
Accordingly, in order to overcome the smoldering problem in textiles, the addition of a smoldering suppressant (SS), which is also referred to herein, interchangeably, as a smoldering suppressing agent, is frequently required.
Choosing the right flame retardant, the right smoldering suppressant and the right application method largely depends on the substrate which has to be protected: the protection of a garment, or the protection of an electrical appliance will inherently pose different requirements and restrictions of the flame retardant used.
Presently, there are four main families of flame-retardant chemicals:                Inorganic flame retardants (such as aluminum oxide, magnesium hydroxide and ammonium polyphosphate);        Halogenated flame retardants, primarily based on bromine and chlorine;        Organophosphorus flame retardants, which are primarily phosphate esters; and        Nitrogen-based organic flame retardants.        
Bromine-containing compounds, in particular aromatic bromines, have been long established as flame retardants but suffer major disadvantages including, for example, high bromine content demand, high dry add-on (binder) demand and a need to add compounds which enhance the flame retardancy (hereinafter termed “synergists”). In addition, application of such FRs on fabrics may result in streak marks on dark fabrics, excessive dripping during combustion of thermoplastic fibers, relatively high level of smoldering and a general instability of the flame retardant dispersion which may prevent a uniform application thereof on the fabric.
Over the years, several antimony-based compounds have been used as flame-retardant synergists, including Sb2O3, Sb2O5 and Na3SbO4 (Touval, I., (1993) “Antimony and other inorganic Flame Retardants” in Kirk Othmer's Encyclopedia of Chemical Technology, Vol. 10, p. 936-954, 4th Edition, John Wiley and Sons, N.Y.). Antimony based compounds are very expensive and are therefore not used on their own, but are used as synergists with other flame retardants. The addition of antimony oxide to halogenated flame retardants increases their efficiency and reduces the amount of additives and/or halogenated FR agent to be used. However, the addition of such synergist is costly and further contributes to the high add-on of the formulation.
Phosphorus-based flame retardants have been a major source of interest to replace halogen compounds. Phosphorus-based flame retardants are characterized by producing environmentally friendly by-products, low toxicity, and low production of smoke in fire, and are highly effective flame retardants for cellulose and cellulose derivatives. Phosphorus-based flame retardant compounds promote dehydration and char formation. However, although cotton fabrics treated with phosphoric acid exhibited good flame retardancy and acceptable tensile strength retention, presently used phosphoric acids have poor durability of the flame retardancy to washing, due to the water solubility of the phosphoric acid. Moreover, it was found that fabrics treated with phosphoric acid turned yellow and became tender when the concentration of phosphoric acid increased (Charuchinda et al., J Sci. Res. Chula. Univ, Vol. 30, No. 1 (2005) 97-106).
Examples of commonly utilized smoldering suppressing agents include urea, melamine and phosphate salts. Furthermore, it has been recently shown that compositions that combine phosphates and halogen display a synergism in flame retardation (E. S. Lee, “Possible Phosphorous Synergy in Polyester-Cotton Fabric Treated with Tetrabromobisphenol A and Diammonium Phosphate”, J. App. Pol. Sci., Vol. 84, 2002, pp. 172-177) and that phosphate and borate compounds are efficient flame retardants in the solid phase during combustion (G. Camino, M. P. Luda, “Fire Retardancy of Polymers: The use of Intumescents”, M. Le Bras, G. Camino, S. Bourbigot, R. Delobel, The Royal Society of Chemistry, 1888, p. 48; R. Dombrowski, “Formulating Flame Retardant Coatings”, Coated Fabrics Technology, Clemson University, 1998).
Phosphate salts are salts of phosphoric acids of varying chain lengths. The most basic phosphoric acid unit is phosphoric acid (H3PO4, also termed monophosphoric acid or orthophosphoric acid) which can undergo dehydration so as to form a series of higher molecular weight condensates. For example, the formation of di-phosphoric acid (H4P2O7, also termed pyrophosphoric acid) and of tri-phosphoric acid is depicted in scheme 1 below:

The dehydration can continue so as to form additional oligomeric condensates of phosphoric acid, which may be collectively described as HO—(HPO3)n—H (or Hn+2PnO3n+1), where n is an integer, as is depicted below:

These higher forms of phosphoric acid are typically termed “polyphosphoric acid(s)” (PPA), and are sometimes also referred to as “superphosphoric acid(s)” (SPA), “phosphoric anhydride(s)” or “condensed phosphoric acid(s)”. In some cases the polyphosphoric acids may further form closed ring systems, which are then termed meta-phosphoric acids.
Examples of metaphosphoric acids are depicted in scheme 2 below:

Polyphosphoric acids appear as mixtures of several oligomers (including ortho, pyro, tri, tetra and higher condensed acids), and are defined by the distribution of the various chain lengths, as well as by the average P2O5 content thereof (by weight) or by the average H3PO4 content thereof by weight (the average H3PO4 content being 1.38 times the average P2O5 content).
Commercially available polyphosphoric acids are classified by two CAS Registry numbers: CAS No. 7664-38-2 which is defined as an acid containing between 50% to 75% P2O5 by weight (corresponding to 70% to 104% H3PO4 by weight), and CAS No. 8017-16-1 which is defined as a compound having at least 80% P2O5 by weight (corresponding to at least 110% of H3PO4 by weight). The percentages defining the constituents in a polyphosphoric acid composition thus describe the relative weight percentages of each constituent.
The relation between the distribution of dehydration products, the percentage of P2O5 and the percentage of H3PO4 is exemplified in Table 1 below.
TABLE 1Percentage Composition in Terms of the Constituent Poly phosphoric Acidstetra-H3PO4P2O5(1 = orthophosphoric acid, 2 = pyrophosphoric acid etc.)High-Tri-Tetra-(%)(%)1234567891011121314Poly.MetaMeta93.067.4100.094.868.799.70.3397.270.496.23.8599.071.791.08.86trace101.573.577.122.10.79102.073.973.625.11.34104.575.753.940.74.860.46107.077.533.550.611.52.680.74trace109.279.122.146.320.37.822.261.020.34111.180.513.838.223.013.06.863.381.671.030.22111.881.012.234.022.714.68.424.362.271.410.56trace112.181.210.932.922.315.09.365.412.851.750.970.360.05113.782.47.3223.019.315.912.38.215.733.892.521.360.910.14trace116.084.03.9211.812.712.010.58.977.996.625.634.543.723.032.461.686.63117.385.02.286.367.3288.177.677.226.936.425.895.274.693.993.8316.9117.785.31.874.736.336.66.666.716.366.115.885.465.074.904.644.3825.6118.986.11.462.813.744.44.524.774.794.934.674.544.674.634.384.1743.50.17120.287.10.831.182.172.53.093.393.463.333.553.473.453.523.263.2461.1trace121.387.90.500.821.561.81.722.032.32.262.072.262.062.201.992.3076.40.420.11123.489.41.881.520.770.60.620.680.540.710.861.030.981.161.231.3786.81.70.41
Phosphate salts which are derived from ammonia (ammonium phosphate compounds) and their metal salts have long been used in agriculture as fertilizers, supplying both nitrogen and phosphorous (Zdukos et al, “Reactions for the Formation of Calcium Ammonium Polyphosphates in Fertilizers”, VINITI, Moscow, 1974; Tonsuaadu et al, “Phosphorus, Sulfur and Silicone”, Vol. 179, No 11, p. 2395, 2004; Lapina et al, “Metal ammonium phosphates: Production of iron and aluminum phosphates”, Nauk SSSR, Otd. Biofiz. Khim. Fiziol. Aktiv. Soedin, 1966, 265-274). Stabilized liquid fertilizer suspensions of calcium ammonium pyrophosphate, comprising crystals smaller than 50 microns, have been prepared from ammonium pyrophosphate (derived from pyrophosphoric acid) and from vitreous calcium silicate, and have been claimed to inhibit the growth of large crystals in the primary component of the fertilizer suspension, i.e. ammonium phosphate (see, U.S. Pat. No. 3,526,495, to Philen).
Other fertilizer compositions comprising calcium ammonium pyrophosphate have been described by Brown and Fraizier in the 60's (U.S. Pat. No. 3,053,623, also in Agriculture and Food Chemistry, Vol. 11, No. 3, p. 214, 1963, and in Agriculture and Food Chemistry, Vol. 12, No. 1 p. 70, 1964).
Aluminum ammonium phosphate has been extensively used in the preparation of amorphous gels, due its potent gelation properties on one hand, and as being an environmentally friendly substance, on the other hand. Thus, for example, a liquid fertilizer that includes aluminum ammonium polyphosphate AlNH4HP3O10, containing 71.2% P2O5, equivalent to 47.6% PO43−, is described by Rilo and Turchin (Zhurnal Prikladnoi Khimii, 1975, 48(1), 199-200). As taught in this publication, this complex was prepared by reacting H3PO4 and ammonia at 200-300° C.
In another study, an amorphous fertilizer system containing aluminum, ammonia, phosphate and water, and between 5% and 33% by weight of P2O5, equivalent to between 3.3% and 22% by weight PO43−, is described by Lapina. L. M and Grishina, L. A, in “Tr. Nauch-Issled. Inst. Udobr. Insektofungits” (1973) No. 221, pp. 56-62. This publication further teaches the advantageous use of amorphous fertilizers, which are capable of containing more nutrients within the amorphous structure, as compared to crystalline fertilizers.
Ammonium taranakite (NH4)3Al5H6(PO4)8.18H2O , (NH4)2AlH(PO4)2.4H2O and NH4Al(PO4)OH.2H2O, are known to be formed from ammonium phosphate fertilizers in the soil. These substances were characterized by Frazier and Taylor, as early as 1956, as containing up to 19% phosphorus, equivalent to 57.9% PO43−, and were synthesized by extremely prolonged reactions (over 3 weeks) at room temperature (see, for example, “Characterization of Taranakites and Ammonium Aluminum Phosphates”, Soil Science Society Proceedings, 1956, 545-547).
U.S. Pat. No. 2,909,451 to Lawler and Vartanian, teaches aluminum phosphate dispersions, in particular those prepared from a water-soluble aluminum salt and at least a stoichiometric amount of a water-soluble orthophosphate. No mention is made in this patent to the phosphate content in the final product. Furthermore, this patent teaches a precipitated aluminum phosphate preparation, which is thereafter dispersed in a liquid medium to obtain a thixotropic composition. This patent is therefore silent with respect to gel preparation.
The use of pyrophosphate salts as flame retardants and smoldering suppressants has also been known in applications which do not require durable agents, namely, applications which require minimal or no stability to UV light, heat, water, detergents, air-pollutants or chemicals. For example, the use of magnesium salts of ammonium pyrophosphate as flame retardants is mentioned in a report by A. A. Gansh and I. M. Kaganskii (Prace Naukowe Akademii Ekonomicznej imienia Oskara Langego we Wroclawiu (1990), 526 107-11).
It is reported that the smoldering suppression effect of phosphate salts is achieved by the release of phosphoric acid upon heating, which promotes char formation instead of flammable volatiles production. These compounds can thus absorb the heat by swelling or foaming and are often employed in intumescing systems (Environmental Health Criteria 192, World Health Organization, Geneva, 1997).
Another class of phosphate salts is the group of ammonium polyphosphates (APP, derived from various polyphosphoric acids), which have been used as smoldering suppressants in plastics and as FR or smoldering suppressants in “nondurable” applications.
The preparation of APP has been described, for example, in U.S. Pat. Nos. 3,342,579 and 3,397,035.
According to the teachings of U.S. Pat. No. 3,342,579, a short-chain, slightly water-soluble APP can be synthesized from polyphosphoric acid in water upon the addition of ammonia gas, at a temperature of 193° C. and at a pressure of 20 atmospheres.
According to the teachings of U.S. Pat. No. 3,397,035, a long-chain, water-insoluble crystalline APP can be synthesized from dry ammonium orthophosphoric acid and urea, at a temperature of 260° C.
An exemplary smoldering suppressant and FR of this family is aluminum ammonium polyphosphate. For example, it has been reported by Mironovitch et al., in “Khimicheskaya Promyshlennost” (1975), 3, pp. 207-209) that AlNH4H2(PO4)2.0.5H2O is formed by a reaction of Al(H2PO4)3 with NH4OH and contains P2O5 in a total amount of 46% and higher, and in an amount of 10% and higher in the aqueous solution. An amount of 46% P2O5 is equivalent to 31% PO43−. It should be noted however that this publication fails to show neither a product nor a process for obtaining such an aluminum ammonium polyphosphate with P2O5 content higher than 46%. Further, the aluminum ammonium polyphosphate referred to in this publication is referred to as a water-immiscible product, which is not suitable in applications requiring durable FRs since it contains miscible ingredients.
Crystalline aluminum APP obtained at high temperature (350° C.) is also described in Averbuch-Pouchot and Guitel (Acta Cryst., B33 (1977), 1436-1438). Other crystalline aluminum APPs are discussed by Golub and Boldog (Russian Journal of Inorganic Chemistry, 19(4), 1974, pages 499-502). U.S. Pat. No. 4,956,172 teaches crystalline aluminum ammonium phosphate, Al2(NH4)OH(PO4)2.2H2O obtained by a reaction of Al(OH)3 with (NH4)2HPO4 or NH4H2PO4, again at elevated temperatures (125-250° C.), for use as a dentifrice polishing agent and filler for plastics.
The use of the described metal complexes of ammonium phosphates as anti-smoldering agents in textile applications has been substantially limited by low fastness to laundry processes. When applied on textiles, the ammonium phosphate additive is washed off within one or few washing cycles. Once this material is washed off, the treated fabric again fails the flammability tests due to prolonged smoldering.
It has been suggested that the instability of APP during laundry is due to the high solubility of ammonium phosphates in water under laundry conditions (basic pH and large amount of water in each cycle), which is further increased by the hydrolysis thereof. It has also been suggested that the ammonium phosphates are incompatible with the acrylic binders used in the coatings, thus resulting in a phase separation which appears as a brittle film.
Furthermore, although many treatments confer wash-resistant flame retardancy in the sense that the retardant will not be removed by laundering, the effect of the detergent used in the washing process is oftentimes neglected, although it may be quite significant. The main concern in this case is that the detergent solution may exchange the ionizable volatile cations on the flame retarded fabric, by sodium ions from the detergent, thus causing (a) an increased solubility of the sodium phosphate salt, and (b) if sufficient ions of the flame retarded fabric are replaced by sodium, the resistance to flaming will be much reduced, as is detailed hereinafter.
Interestingly, it has been found that the flame retardancy effectiveness of the phosphate complexes largely depends on the counter ion. For example, the effectiveness of a metal phosphate as a flame retardant of a cellulose fiber is in an order of magnitude less than that of an ammonium phosphate (J. W. Lyons, The Chemistry and Uses of Fire Retardants, Wiley-Interscience, New York, 1970). Table 2 below presents exemplary values of the amount of substance needed to render cellulose nonflammable as a function of the phosphate counter ion (W. H. Perkin, J. Ind. Eng. Chem., 5, 57 (1913)).
TABLE 2SubstanceParts/100 Parts celluloseAmmonium phosphate4.5Sodium phosphate30.0Aluminum phosphate30.0Calcium phosphate30.0Magnesium phosphate30.0
Thus, the laundry fast flame retardant and/or smoldering suppressant has to be not only chemically resistant to water, but also, at least to some degree, immune to ion exchange.
In an attempt to improve the durability of phosphate salts, chitosan, which is a natural biopolymer containing an amino group, was recently co-applied with sodium polyphosphate onto cotton fabric (Charuchinda et al. 2005, supra), expecting that the co-application of chitosan would impart a synergistic activity with the phosphoric acids by enhancing the formation of intumescent chars. However, results showed that while the flame retardancy of cotton treated with this mixture slightly increased, and a film layer was observed, after only one mild washing (for 30 minutes at 50° C.) the flame retardancy was almost identical to that of the untreated fabric.